Due to request for low abrasion on rotational elements to achieve an extended life and for low extent of noise, fluid dynamic bearings (FDB) have been used in conventional fan motors.
FIG. 6 depicts a fan motor using an FDB unit. The motor comprises a shaft 80 extending through a bearing sleeve 81 with a clearance space formed therebetween. The clearance space is filled with lubricant oil that provides a medium through which a dynamic fluid pressure field may be generated. Relative rotation between the bearing sleeve 81 and the shaft 80 is required to set up the dynamic fluid pressure field. The bearing sleeve 81 supports radial load by metal-to-metal contact when there is no relative motion. During normal operation, the spinning of the shaft 80 sets up a steady pressure field around the clearance space that pushes the shaft 80 and the bearing sleeve 81 apart and thus prevents metal-to-metal contact. To obtain an improved dynamic pressure field, grooves 82 are formed on the inner surface of the bearing sleeve 81.
The bearing sleeve 81 is disposed in a housing 83. A ventilating passage 85 is formed between the outer periphery of the bearing sleeve 81 and an inner surface of the housing 83. The ventilating passage 85 has a vertical section and a horizontal section. This ventilating passage 85 allows air to escape the bearing sleeve 81 when the shaft 80 enters the bearing sleeve 81. However, the fluid dynamic bearing system is cooperatively formed by two components, i.e., the housing 83 and the bearing sleeve 81. To ensure the dynamic fluid pressure, the two components must be precisely produced and then assembled together. This structure is complicated and necessitates a highly manufacturing cost.
Referring to FIG. 7, for the sake of clarity, a portion of the inner surface of the bearing sleeve 81 is unfurled to shown a groove pattern having the grooves 82. Each groove 82 is V-shaped and has two branches 87a, 87b having a common intercrossing area 88. A top edge of the groove pattern faces to the outside of the bearing sleeve 81, so the lubricant oil in the groove at the top edge exposes to atmosphere. Sealing measures must be taken to prevent leakage of the lubricant oil at the top edge. When there is no relative rotation, sealing is conventionally provided by surface tension capillary seals in which a lubricant-air interface provides the surface force. See U.S. Pat. No. 5,112,142 for relevant technology. When the shaft 80 rotates, the lubricating oil is driven from ends of the branches 87a, 87b to the intercrossing area 88 to generate a high pressure. At the same time, because a part of the lubricating oil is moved to the intercrossing area 88, the lubricating oil remaining near the ends of the branches 87a, 87b generates a very low pressure. This low pressure is required to be even lower than the outside atmosphere pressure so that the lubricant oil at the top edge does not flow outside. However, if the lower pressure generated by the lubricating oil at the top edge is very close to the outside atmosphere pressure, the lubricating oil is still possible to leakage, when the motor is subject to vibration during use or the motor is used in a location where the outside atmosphere pressure is lowered. Therefore, the lower pressure generated by the lubricating oil at the top edge is desired to be low enough.
For the foregoing reasons, there is a need for a fluid bearing having a simple structure with low cost. There is also a need for a fluid bearing having an improved capability to prevent leakage of lubricating oil.